For many years the airports have been equipped with the Instrument Landing System known as the ILS. However, this landing system is about to be replaced by a new Microwave Landing System (MLS) which has recently received virtually worldwide acceptance.
In view of the essential nature of aircraft landings, often considered to be the most critical of ordinary maneuvers, it is important to have a monitoring and back-up system that provides an independent check during in-flight approaches to insure the absolute reliability of the data being provided by the principal landing system. This would be in addition to the usual ground-based monitors which are used for checking course alignments, signal strengths, etc. of the signals radiated from the ground-based landing system components. Although other types of navigation techniques are constantly in use in an aircraft to check on the accuracy of the landing system, none provides the high degree of reliability required during final approach.
A detailed discussion of the means by which monitoring systems may be used for the critical landing operation is provided in my U.S. Pat. No. 4,635,064. Also included therein is an overview of MLS.